expose information about negotiated connection settings

h2 currently permits configuring most of the HTTP/2 protocol settings that are negotiated with the remote peer; for example, the client builder can configure settings like maximum frame size, maximum concurrent streams, etc.

However, once a connection is established, there's no way to access information about the settings that were negotiated with a remote peer, besides information about stream flow control. For example, in my particular use case, we would like to be able to record the max concurrent streams limit set by the server after initiating a client connection. It would be great to add some kind of API for querying connection-level settings that were received from a remote peer.